Gas laser circulation system

ABSTRACT

A gas circulation system for a laser comprises a reaction vessel comprising an empty tube which may be heated to an appropriate temperature, and means for circulating the gas from the laser through the reaction vessel so that the dissociation of the gas which occurs in use is reversed.

United States Patent [191 Hepburn GAS LASER CIRCULATION SYSTEM [75]Inventor: William Dunn Hepburn, Edinburgh,

Scotland [73] Assignee: Ferranti Limited, Hollinwood,

Lancashire, England [22] Filed: June 19, 1972 [21] Appl. No.: 263,948

[52] US. Cl. 331/945 [51] Int. Cl. H01s 3/02 [58] Field of Search331/945 [56] References Cited UNITED STATES PATENTS 3,391,281 7/1968Eerkens 33 l/94.5

[ Jan. 29, 1974 3,596,202 7/1971 Patel 33 l/94.5 3,634,778 l/l972Melikian et 331/945 3,566,297 2/1971 Blackman, Jr. 331/945 PrimaryExaminerWilliam L. Sikes Attorney, Agent, or Firm-Cameron, Kerkham,Sutton, Stowell & Stowell [5 7] ABSTRACT A gas circulation system for alaser comprises a reaction vessel comprising an empty tube which may beheated to an appropriate temperature, and means for circulating the gasfrom the laser through the reaction vessel so that the dissociation ofthe gas which occurs in use is reversed.

5 Claims, 1 Drawing Figure GAS LASER CIRCULATION SYSTEM This inventionrelates to gas lasers and particularly to such lasers of the high-powercarbon-dioxide type.

Certain high power gas lasers use as the active medium a mixture ofcarbon dioxide, nitrogen, and helium. It has been found that theperformance of such a laser falls off if the gas mixture is allowed toremain in the tube, and for this reason it is usual to replenish the gasmixture, continously passing the mixture into the laser tube and ventingit to the atmosphere. The disadvantage with this technique is thathelium is an expensive material, and in the larger lasers a considerablequantity of helium is required.

It is believed that the fall-off in the performance of teh laser is due,at least in part, to a change in the chemical composition of the gasmixture. The most probable reaction is represented by the followingreversible equation, though other products containing one or more of theelements carbon, nitrogen or oxygen may be formed.

CO +N +He CO+N O+He.

The carbon dioxide and nitrogen are changed, at least in part, by theeffects of the electric discharge used to excite the laser, and formcarbon monoxide and nitrous oxide. The helium is not affectedchemically.

It is an object of the invention to provide a gas circulation system fora laser such that the rate of replenishment of the gas in the laser issubstantially reduced.

It is a further object of the invention to provide a laser having such agas circulation system.

According to the present invention there is provided a gas circulationsystem for a laser comprising a reaction vessel, means for circulatingthe gas used in the laser through the reaction vessel, and means formaintaining the reaction vessel at a temperature at which at least someof the dissociation of the gas occurring in the laser is reversed.

An embodiment of the invention will now be described with reference tothe accompanying drawing. This shows, in schematic form, a laser tubeand the associated gas circulation system.

Referring now to the drawing, the laser itself is shown in a simplifiedform as comprising a tube having a pair of end mirrors 11 and 12, andbeing provided with two discharge electrodes 13 and 14. The remainder ofthe drawing shows the gas circulation system. Gas is extracted from thelaser discharge tube by means of a pump 15, and is passed through anoil-mist filter 16A and a silica-gel trap 16B to remove any water vapouror contamination due to the pump itself. After passing through thefilter the gas mixture passes through a reaction vessel in the form ofan otherwise empty alumina tube 17 heated by a heating element 18, andthence through a further silica-gel trap 16B if required, to remove anywater vapour or organic vapours. The gas mixture then returns through athrottle valve 19 to the laser discharge tube 10. The circuit betweenthe throttle valve 19 and the pump via the laser tube 10 is maintainedat the low pressure required to operate the laser, in the range 10 to100 torr whilst the remainder of the circuit is at a higher pressure,usually at or slightly above atmospheric pressure.

in order to allow for gas leakage from the system sources of the threegases are provided as shown at 20,

The oil-mist filter is necessary only if the pump is of av type whichmay introduce oil contamination.

The rate of usage of the make-up gases from sources 20, 21 and 22 willbe very small compared with the quantities required when the circulatinggas mixture is exhausted direct to the atmosphere. It may be desirableto use flow-meters at various points in the circulation system, thoughthese have not been shown in the drawing. The gases may, of course, bepre-mixed and drawnfrom a single cylinder.

in operation the heating element 18 is arranged to maintain the aluminatube 17 at a temperature of about 700C. At this temperature there-combination reaction occurs at the required rate. It may be necessaryto cool the gases before passing them back into the laser dischargetube.

The reaction vessel may be made of other materials than alumina. Forexample, silica and mullite are possible alternatives. Generally thematerial must be one which is inert to the gases passing through it.

The use of a pressure equal to or greater than atmospheric pressure inthat part of the system which includes the reaction vessel ensures thatany leaks which occur involve the emission of gas from the system. Thisis to be preferred to the possible ingress of contamination from outsidewhich would occur if the pressure was less than atmospheric. it shouldalso be remembered that the higher the pressure the higher is thereaction rate, and the relatively high pressure avoids the necessity touse a catalyst in the reaction vessel.

What I claim is:

l. A gas laser which includes a discharge tube containing a mixture ofgases, means for exciting the gas mixture to cause laser action anddissociation of the gas in the discharge tube, and a closed-circuit gascirculation system comprising a reaction vessel, means for circulatingthe gas mixture through the reaction vessel, means for maintaining thegas mixture in the reaction vessel at a pressure greater than that inthe discharge tube, and means for maintaining the reaction vessel at atemperature at which at least some of the dissociation of the gasoccurring in the discharge tube is reversed;

2. A gas circulation system as claimed in claim 1 in which the gas inthat part of the circulation system which includes the reaction vesselis maintained, in operation, at a pressure at least equal to atmosphericpressure.

3. A laser as claimed in claim 1 in which the reaction vessel is madefrom a material selected from the group consisting of alumina, silicaand mullite.

4. A laser as claimed in claim 1 in which the reaction vessel ismaintained, in operation, at a temperature of the order of 700C.

5. A laser as claimed in claim 1 which includes means for replenishingany of the constituents of the gas mixture.

1. A gas laser which includes a discharge tube containing a mixture of gases, means for exciting the gas mixture to cause laser action and dissociation of the gas in the discharge tube, and a closed-circuit gas circulation system comprising a reaction vessel, means for circulating the gas mixture through the reaction vessel, means for maintaining the gas mixture in the reaction vessel at a pressure greater than that in the discharge tube, and means for maintaining the reaction vessel at a temperature at which at least some of the dissociation of the gas occurring in the discharge tube is reversed.
 2. A gas circulation system as claimed in claim 1 in which the gas in that part of the circulation system which includes the reaction vessel is maintained, in operation, at a pressure at least equal to atmospheric pressure.
 3. A laser as claimed in claim 1 in which the reaction vessel is made from a material selected from the group consisting of alumina, silica and mullite.
 4. A laser as claimed in claim 1 in which the reaction vessel is maintained, in operation, at a temperature of the order of 700*C.
 5. A laser as claimed in claim 1 which includes means for replenishing any of the constituents of the gas mixture. 